Field of the Invention
The present invention relates to a method for trimming a rectangular container and an apparatus trimming a rectangular container, and in particular, to a method for trimming a rectangular container with a rectangular cross section and an apparatus trimming such a rectangular container.
Description of Related Art
A rectangular container shaped by drawing, drawing-ironing, or the like needs trimming, in a post-process, of an uneven edge of an opening in the rectangular container formed by these types of processing. Various trimming methods are known (Patent Literature 1 to 3).
Besides the methods in Patent Literature 1 to 3, a method for trimming a rectangular container is known in which short-side wall portions involving low cut loads are first removed (FIG. 13A) and in which long-side wall portions involving high cut loads are then removed (FIG. 13B), as depicted in FIG. 13. In such four-side cut trimming, a step is likely to be formed at the boundary between the short-side wall portion and the long-side wall portion on a trimmed end surface, as indicated by a dashed line in FIG. 14. Furthermore, the wall portions are cut by stroking a punch cutter on each side, and thus, at least four processes are needed. Moreover, a transfer press is used for cutting, and thus, a production rate is limited to 20 to 50 cpm (cans per minute: the number of cans produced per minute) in view of the life of a cam.
In view of a reduction in processes, a trimming apparatus is known by which a container is trimmed by circumferentially moving an inner-frame cutter (punch) as described in Patent Literature 2 and Patent Literature 3. However, with such a trimming apparatus, a removal portion and a part of a product portion (remaining portion) remain connected together until the end of the process. Thus, for a rectangular container with a rectangular cross section as depicted in FIG. 13, the removal portion is likely to move uncontrollably as the cutter moves, imposing a heavy load on the boundary between the removal portion and the product portion. As a result, stress is concentrated at the boundary, which gradually begins to split. This leads to a crack or burr in the product.    [Patent Literature 1] Japanese Patent Application Laid-open No. S49-45491    [Patent Literature 2] Japanese Patent Application Laid-open No. S44-8357    [Patent Literature 3] Japanese Utility Model Application Laid-open No. S59-85623